1. Field of the Invention
The invention relates to a strain-relieved leading-in connection for signal cables with twisted wire pairs. In particular, the present invention concerns a strain-relieved leading-in connection for signal cables with a cable jacket and with several twisted wire pairs, respectively an apparatus or a connection element of the plug or socket type with such a leading-in connection, where clamping means are provided for co-operation with the cable for the strain-relieving of a closely disposed termination area for the wires in the cable. Regardless of whether the ends of the wires are terminated by soldering or clamping, it will be unfortunate if forces arising on the cable shall be absorbed solely by these assemblies. The same applies for current supply cables, also including those for power lines where, e.g. in connection with input plugs, it is quite traditional to use strain-relieving by firm clamping of the cable in the relevant lead-in connection.
2. Description of Related Art
It must be ascertained that precisely this form of relieving technique has by and large been practiced in the signal cable area, i.e., by a simple clamping of the cable between opposing clamping strips, which with good tightening provides an excellent strain relief, in that the clamped-together cable jacket, which normally consists of flexible plastic, will pack well around the leads and hereby secure these with great friction. However, it is precisely with signal cables that this gives rise to distinct problems, which will now be explained in more detail with reference to FIGS. 1 and 2 of the drawing:
FIG. 1 is a schematic view of a cross-section of a signal cable with an outer cable jacket 2 and a number of internal wire pairs, designated 4a-d. 
Typically, each pair of cables is twisted with a quite definite pitch which is different from the others, and the pairs are twisted around one another throughout the whole length of the cable. In principle, this provides a perfect electrical balance between the wires in the individual pairs and between the pairs.
FIG. 2 shows that, when the same cable is terminated in a plug, by today""s standard, the round cable will be pressed into a common lead-in passage where, typically by using common clamping means 6, the jacket 2 with the wire pairs 4 are squeezed in a random and often quite unfortunate manner, in that the twist conditions are changed to a dramatic extent. Note, e.g., pairs 4a and 4c, where the possibility is created for a very inductive coupling, the reason being that the pair twisting has been ruined. The signal carried by the cable is hereby brought into great unbalance, in that undesirable signal coupling is generated between these pairs. Moreover, the impedance seen in the strain relief will be changed considerably because of the ruined pair twisting. This results in undesirable signal reflection from the cable relieving point.
In FIG. 3 is shown another known way of providing strain-relief for signal cables. Use is made here of a lead-through opening with an annular clamping arrangement 6 which can secure the cable by clamping around the cable jacket. This will result in less disturbance of the twist conditions, but on the other hand, will give rise to an unfortunate tight squeezing together of the twisted wire pairs. The cable Jacket can be secured in a reasonably effective manner by a moderate twisting together, but as indicated in FIG. 3, traction and/or torsion in the cable with thus give rise to longitudinal movements of the wire pairs, so that the termination points are loaded by these movements. Consequently, the terminations become unstable, regardless of whether use is made of soldering, crimping, blade contacts or the like.
The object of the invention is to provide a lead-in connection which does not display the above-mentioned disadvantages, and according to the invention this can be achieved by dividing the lead-in area into mutually separated lead-in passages for the individual wire pairs, in that each of these passages is provided with clamping means for the clamping of the wire pairs which are led through them. It will be seen that a fixing of the cable jacket is hereby more or less done away with, but on the other hand, it is achieved that the wire pairs will be completely separated and be individually and effectively secured, so that the termination areas are spared for undesirable influences on the cable. The individual wire pairs naturally cannot absorb the same high forces as those which can be absorbed by a strong clamping of the whole of the cable, but by virtue of the surface pressure against the insulation layer of the wires in the respective wire pairs, a particularly good holding effect can, however, still be created, and which in by far the majority of cases will be fully adequate.
In practice it will be necessaryxe2x80x94and even directly desirablexe2x80x94for the wire pairs to be led to lead-in positions -in which they are mutually separated by an even greater distance than inside the cable. The electrical couplings arising between the pairs will hereby be automatically weakened, and -it will thus be of less importance that the regular twisting of the individual wire pairs can be disturbed quite locally in the individual lead-in passages.
The invention will now be explained in more detail with reference to the drawing.